1. Field of the Invention
This invention relates generally to the operation of a delay/amplitude equalizer device, and more particularly to apparatus for providing signals needed to adjust the tap weights of taps in a transversal delay line.
2. Description of the Prior Art
The necessity for the correction of constant frequency-independent frequency shifts, generally referred to as frequency offset, is well known in the prior art. For example, sine waves of 500 Hz, 1500 Hz and 2500 Hz (i.e., a fundamental, third and fifth harmonic) can be combined, when the phases and relative amplitudes are correctly specified to approximate a square wave. However, when the frequency offset is 5 Hz, for example, the resulting signals of 505 Hz, and 1505 Hz and 2505 Hz do not provide for a simple reconstruction of a square wave. For certain types of transmitted information, a distortion of this type can produce intolerable error.
In a delay/amplitude equalizer device, corrections are performed by comparing a received reference signal with a similar generated internal reference signal. The delay/amplitude equalizer device provides apparatus for reconstructing the received signal so that spectrum of the received signal is approximately equivalent with the spectrum of the internally generated signal. In particular this spectral reconstruction is typically accomplished by correctly weighting taps on a transversal filter tapped delay line.
In order to eliminate the offset frequency error in a received signal, an offset frequency signal can be introduced into the reference signal, or the offset frequency components can be eliminated from the received signal. Further, the received signal plus reference signal with offset frequency added combination or the corrected received signal plus reference signal can be utilized by the circuits controlling the delay tap weights to provide desired correction.
In the prior art, the offset frequency has typically been eliminated from the received signal for providing signals determining the weighting of the filter taps. This procedure has several disadvantages. Apparatus required to implement this correction (i.e. such as single sideband filters) are typically complex and expensive. Furthermore, the filter can introduce delay distortion which degrades the received signal and complicates the equalization process. In addition, the phase-locked loop can be sensitive to noise and to (external or internal) phase instability. Consequently, the signal integrity can be compromised. Finally, the correction apparatus, such as filtering apparatus, is still present even when the offset correction is not required.
The offset frequency can be corrected by providing digitized circuits and converting the incoming signals to digitally encoded signals. The digitally encoded signals can then be processed and in this manner the delay distortion can be minimized. However, the complexity resulting from the requirement for a single sideband filter and a low pass band filter remains.
In applications such as a delay/line equalizer, the portion of the circuit used to achieve equalization can generally be utilized on a small portion of the total operating time. After the controller has serviced one equalizer, it is available to service another equalizer. If the circuitry used for correcting the offset frequency can be changed from the equalizer to the controller, a proportionate saving in apparatus can be achieved. This can be done by introducing the same amount and polarity of offset into the reference signal as exists in a received signal channel. This will have the further advantage that when the controller is no longer required by a given equalizer, the impairment, caused by the offset correction, is eliminated.
It is therefore an object to provide an improved delay/amplitude equalizer.
It is a further object to provide apparatus for controlling a plurality of delay/amplitude equalizers which can be shared among the plurality of equalizers.
It is a more particular object of the present invention to provide a reference signal with an offset frequency component determined by the offset frequency component of a received signal.